1. Field of the Invention
The present invention relates to a nonreducing dielectric ceramic, and a monolithic ceramic capacitor using the same.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication Nos. 60-131708, 63-126117, 5-9073, 5-217426, 10-330163 and 10-335169 disclose (Ca1-xSrx)m(Zr1-yTiy)O3-type dielectric ceramic materials as nonreducing dielectric ceramic materials which exhibit excellent dielectric characteristics and do not become semiconductive even when internal electrodes provided therefor are composed of an inexpensive base metal such as nickel (Ni), copper (Cu), etc., and baking is performed in a neutral or reducing atmosphere having low oxygen partial pressure.
By using these dielectric ceramic materials, dielectric ceramics which do not become semiconductive even when baking is performed in a reducing atmosphere can be formed. Moreover, the production of monolithic ceramic capacitors having internal electrodes composed of a base metal such as nickel (Ni) or copper (Cu) has become possible.
However, in the nonreducing dielectric ceramics disclosed in the above-described Japanese Unexamined Patent Application Publication Nos. 60-131708 and 63-126117, raw materials, i.e., calcium carbonate (CaCO3), strontium carbonate (SrCO3), titanium dioxide (TiO2) and zirconium dioxide (ZrO2) are calcined at the same time as manganese dioxide (MnO2), which is a secondary component, and silicon dioxide (SiO2), which is a mineralizer, so as to make a ceramic having a main component satisfying the formula (Ca1-xSrx)m(Zr1-yTiy)O3. As a consequence, the resulting calcined material powder has not only peaks characteristic of a perovskite crystal phase which is the primary crystal phase, but also peaks indicating crystal phases not of the perovskite crystal phase. When the dielectric ceramic is formed by sintering one of these calcined material powders in a reducing atmosphere, crystal phases not of the perovskite-structured primary crystal phase (i.e., different phases) remain in the resulting dielectric. When the thickness of an element is reduced to manufacture a miniaturized high-capacitance monolithic ceramic capacitor, the performance thereof in a high-temperature loading lifetime test is degraded since these different crystal phases have inferior thermal resistance.
Japanese Unexamined Patent Application Publication Nos. 63-126117, 5-9073, 5-217426, and 10-330163 disclose nonreducing dielectric ceramics containing lithium (Li) or boron (B) in their additive glasses. Because Li and B readily evaporate at high temperatures, fluctuations in furnace temperature and unevenness of the atmosphere result in fluctuation in the amount of Li or B evaporating and the evaporation time. Thus, the characteristics such as electrostatic capacitance of the resulting capacitors are irregular.
Japanese Unexamined Patent Application Publication No. 10-335169 discloses a nonreducing dielectric ceramic comprising a main component represented by the formula [(CaxSr1-x)O]m[(TiyZr1-y)O2], manganese oxide, aluminum oxide, and a secondary component represented by the formula [(BazCa1-z)O]vSiO2. The nonreducing dielectric ceramic does not contain components which readily evaporate during baking. Consequently, the ceramics show greater reliability in a high-temperature loading lifetime test and exhibit less irregularity in performance. The nonreducing dielectric ceramic indeed shows some improvement in insulation-resistance in a high-temperature loading lifetime test but has a significant proportion of crystal phases which are not of the perovskite primary crystal phase. As a result, degradation of insulation-resistance is observed in a moisture-resistance loading test.
Recently, the demand for smaller monolithic ceramic capacitors having large capacitance has required thin yet highly reliable dielectric ceramic layers. In order to meet this need, a highly reliable dielectric ceramic material capable of forming thinner layers and a small, yet highly reliable, monolithic ceramic capacitor having large capacitance at high temperatures and high humidity is desired.
Accordingly, it is an object of the present invention to provide a nonreducing dielectric ceramic including a main component having a perovskite crystal phase, the main component satisfying the formula
(Ca1-a-b-cSraBabMgc)m(Zr1-w-x-y-zTiwMnxNiyHfz)O3
wherein 0xe2x89xa6a less than 0.5, 0xe2x89xa6b less than 0.5, 0xe2x89xa6c less than 0.05, 0xe2x89xa6a+b+c less than 0.5, 0.98xe2x89xa6m less than 1.03, 0xe2x89xa6w less than 0.6, 0xe2x89xa6x less than 0.05, 0xe2x89xa6y less than 0.05, 0xe2x89xa6z less than 0.3, 0xe2x89xa6x+yxe2x89xa60.05, and 0xe2x89xa6w+x+y+z less than 0.6 and at least one type of compound oxide selected from one of the group consisting of (Si, T)O2xe2x80x94MOxe2x80x94XO wherein T is at least one element selected from Ti and Zr, MO is at least one selected from MnO and NiO, and XO is at least one selected from BaO, SrO, CaO and MgO and (Si, T)O2xe2x80x94(Mn, Mxe2x80x2)Oxe2x80x94Al2O3 wherein T is at least one of Ti and Zr, and Mxe2x80x2 is at least one selected from Ni, Ba, Sr, Ca and Mg. The proportion of the intensity of the maximum peak of a crystal phase not of the perovskite crystal phase to the intensity of the maximum peak assigned to the perovskite crystal phase appearing at 2xcex8=25 to 35xc2x0 is about 5% or less in a CuKxcex1 X-ray diffraction pattern.
Preferably, the compound oxide (Si, T)O2xe2x80x94MOxe2x80x94XO represented by the formula xcex1(Si1-xcexc-xcexdTixcexcZrxcexd)O2xe2x80x94xcex2(Mn1-"xgr"Ni"xgr")Oxe2x80x94xcex3XO, wherein xcex1, xcex2 and xcex3 are molar percent and XO is at least one of BaO, SrO, CaO and MgO satisfies the relationships 0xe2x89xa6xcexc less than 0.5, 0xe2x89xa6xcexd less than 0.7, 0xe2x89xa6"xgr"xe2x89xa61.0, 0xe2x89xa6xcexc+xcexdxe2x89xa60.7. The (Si1-xcexc-xcexdTixcexcZrxcexd)O2 content, the (Mn1-"xgr"Ni"xgr")O content and the XO content in the compound oxide preferably lie within the region surrounded by points A (xcex1=25.0, xcex2=75.0, xcex3=0), B(xcex1=100.0, xcex2=0, xcex3=0), C (xcex1=20.0, xcex2=0, xcex3=80.0), and D (xcex1=5.0, xcex2=15.0, xcex3=80.0) including the lines AB, AD, and DC, and excluding the line BC is used as the compound oxide in a ternary diagram.
Preferably, the compound oxide (Si, T)O2xe2x80x94(Mn, Mxe2x80x2)Oxe2x80x94Al2O3 represented by the formula xcex1(Si1-xcexcTxcexc)O2xe2x80x94xcex2(Mn1-xcexdMxcexd)Oxe2x80x94xcex3Al2O3 wherein xcex1, xcex2, and xcex3 are molar percent, T is at least one of Ti and Zr, and Mxe2x80x2 is at least one of Ni, Ba, Sr, Ca and Mg, satisfies the relationships 0xe2x89xa6xcexc less than 0.5 and 0xe2x89xa6xcexd less than 0.5. The (Si1-xcexcTxcexc)O2 content, the (Mn1-xcexdMxcexd)O content and the Al2O3 content in the compound oxide preferably lie within the region surrounded by points A (xcex1=80.0, xcex2=20.0, xcex3=0), B(xcex1=10.0, xcex2=90.0, xcex3=0), C (xcex1=10.0, xcex2=20.0, xcex3=70.0), D (xcex1=30.0, xcex2=0, xcex3=70.0), and E (xcex1=80.0, xcex2=0, xcex3=20.0) including the lines AE, BC and CD and excluding the lines AB and ED in a ternary diagram.
The present invention also provides a monolithic ceramic capacitor including a plurality of dielectric ceramic layers, internal electrodes provided between the plurality of dielectric ceramic layers and external electrodes electrically connected to the internal electrodes. Each of the plurality of dielectric ceramic layers is formed of the above-described nonreducing dielectric ceramic in accordance with the present invention. The internal electrodes are formed of a base metal as the main component.
The monolithic ceramic capacitor may be provided with plating layers on the surfaces of the external electrodes.
The base metal is preferably one selected from the group consisting of Ni, a Ni alloy, Cu and a Cu alloy.
The nonreducing dielectric ceramic in accordance with the present invention exhibits a high specific resistance of 1013 xcexa9xc2x7cm or more and a low dielectric loss of 0.1% or less. The rate of change in electrostatic capacitance is within xe2x88x921000 ppm/xc2x0 C. The performance thereof in a high-temperature loading lifetime test and moisture-resistance loading test is highly reliable. Moreover, irregularities in the characteristics thereof are reduced since substances which evaporate during sintering are not contained therein.
By using the nonreducing dielectric ceramic of the present invention, the production of monolithic ceramic capacitors having internal electrodes composed of an inexpensive base metal becomes possible. As the base metal, not only elemental nickel and a Ni alloy but also elemental copper and a Cu alloy having a superior high-frequency performance can be used to manufacture small high-performance monolithic ceramic capacitors.
The nonreducing dielectric ceramic of the present invention can be applied to temperature-compensating capacitors and microwave dielectric resonators. It can also be used as the material for small-size high-capacitance monolithic ceramic capacitors since the layers formed therefrom are thin. The scope of the industrial application is significantly wide.